In commonly assigned U.S. Pat. Nos. 4,590,842 and 4,715,261 and in the co-pending, commonly assigned U.S. Pat. application Ser. No. 809,071 filed Dec. 13, 1985, now abandoned in favor of U.S. application Ser. No. 07/252,551, filed Oct. 3, 1988 (now U.S. Pat. No. 4,974,487) and Ser. No. 061,214, filed June 12, 1987, there are disclosed projectile accelerators wherein a capillary discharge is formed between a pair of spaced electrodes at opposite ends of a tube having a capillary passage. The term "capillary passage and/or discharge" as used in the prior art patents, applications and herein, refers to a passage and/or discharge having a length to diameter ratio of at least 10:1. In all of these prior art devices, the capillary discharge is formed in a dielectric tube, preferably formed of a hydrocarbon, particularly polyethylene. In response to a discharge voltage between the electrodes, a relatively high impedance plasma, that is, an impedance matched to a high voltage power supply for the discharge, forms in the capillary passage. The plasma in the passage fills the passage, causing material to be ablated from the dielectric wall in response to the high temperature of the plasma that is convectively transferred to the wall, as well as in response to radiation of the plasma being incident on the wall. The dielectric wall tube is preferably a hydrocarbon because hydrogen atoms become dissociated from the carbon atoms, to produce copious amounts of high pressure, high temperature hydrogen. Hydrogen is a preferred material for the high pressure source because of its high sound speed associated with the low atomic weight thereof.
In the prior art devices, high temperature, high pressure plasma gas flows longitudinally of the discharge and of the passage through an aperture defined by an electrode at one end of the capillary passage. The gas flowing longitudinally from the capillary passage through the aperture produces a high pressure, high velocity gas that accelerates a projectile to a high velocity.
In U.S. Pat. application Ser. No. 809,071, the high pressure, high temperature plasma is incident on a fluid such as water. The water interacts with the plasma to cool the plasma and prevent ablation from a barrel bore wall through which the projectile is accelerated by the high pressure gas resulting from the interaction of the plasma with the liquid. In U.S. Pat. application Ser. No. 061,214, hydrogen is produced by interacting the plasma flowing through the orifice with a metal hydride and some other material to produce high pressure hydrogen. The plasma is also cooled since it interacts with a cooling agent, e.g. water, while an exothermal reaction is occurring.
In U.S. Pat. application Ser. No. 809,071, the pressure acting on the rear of the projectile is maintained constant while the projectile is accelerated through the barrel bore even though the volume of the barrel bore between the plasma source orifice and the projectile increases. This result is attained by increasing the electric power applied to the capillary discharge in a substantially linear manner as a function of time.
The prior art devices wherein plasma flows longitudinally out of an aperture at the end of a capillary passage have performed capably to accelerate projectiles to very high speeds, e.g., approximately 5 km per second for small projectiles and in excess of 2 km per second for relatively large, metal projectiles that have pierced one inch armor with holes about two inches in diameter. However, the prior art structures are relatively long because of the need for a fuel container, e.g., containing the metal hydride, to be downstream of the plasma source outlet or orifice. Because the plasma flows longitudinally through the orifice, the fuel must be in a container that extends longitudinally from the plasma source as disclosed in co-pending application Ser. No. 809,071.
The efficiency of the prior art device has been limited to approximately 38% because the plasma power flux must flow through a choke throat at the end of the capillary passage. It is most desirable for only very low atomic weight materials having high sound speeds, such as hydrogen, to move in the capillary passage. There are also substantial limitations on the amount of power that can longitudinally flow out of an opening at the end of a capillary passage. Such power limit is equal to ##EQU1## where .gamma. is the ratio of the specific heat of the gas at constant pressure to the specific heat of the gas at constant volume, typically equal to 1.25 for the plasma flowing through the orifice,
P is the plasma pressure at the orifice, i.e., throat, of the capillary discharge, PA1 v is velocity of the plasma flowing through the throat and PA1 A is the throat area.
The maximum plasma pressure, typically equal to about 7 kilobars, is limited by the materials forming the plasma throat. The plasma velocity at the throat is limited by the plasma temperature to a value of approximately 10 km per second; the maximum area of the plasma throat is limited by the plasma electrical resistance to a value of about 1.5 cm.sup.2. From these stated maximum values for PvA and a 1.25 value of .gamma., the typical maximum plasma power from a single prior art axial flow power source is limited to about 5.times.10.sup.9 watts. To accelerate projectiles in large bore guns, where tens of gigawatts are required, multiple sources of the prior art type must be employed.
It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for providing a high pressure gas that is particularly suited to accelerate a projectile.
Another object of the present invention is to provide a new and improved electrothermal gun.
A further object of the present invention is to provide a new and improved electrothermal gun having greater efficiency than prior art electrothermal guns.
Still an additional object of the present invention is to provide a new and improved electrothermal gun having a cartridge that is relatively short in length, even though it may include a fuel source downstream of a plasma generator.
Still an additional object of the present invention is to provide an electrothermal gun that is particularly adapted for use in connection with large bore projectile acceleration.
An additional object of the present invention is to provide an electrothermal gun having multiple plasma sources arranged in a compact manner enabling facile mixture of gases from the several sources.